Pressure-sensitive adhesives can be applied to a substrate as pre-cured adhesive tapes which may be die-cut to the required shape. While this technique is convenient for many technical applications and does, in particular, usually not require any in-situ curing step, it may be disadvantageous in other technical applications for the following reasons:    Two-sided adhesive tapes usually require the use of one or more release liners which have to be discarded upon application of the adhesive tape. When die-cutting the adhesive tape to a desired shape, the excess tape has to be discarded as well.    Especially in the electronics industry the shapes of the substrates to be adhered, tend to get smaller and more complicated so that the die-cutting of the adhesive tape tends to become more difficult and technically demanding.
In view of this it has been suggested to use liquid adhesives which can be applied, for example, by a printing process. U.S. Pat. No. 6,180,200 B1 discloses, for example, a hybrid free-radically and cationically curable pressure-sensitive adhesive composition which can be applied by screen printing and is used for bonding digital versatile disks (DVD). The composition comprises acrylate oligomers and, optionally, acrylate monomers which are copolymerized with a cationic epoxy resin system. The acrylate component of the composition is cured by UV irradiation. Then the two DVD halves are pressed together and the epoxy component of the composition continues to cure cationically. The “dark” cure time of the epoxy component typically is 24 h which is, however, not acceptable for many applications.
JP 2001/226,645-A discloses a composition comprising an acrylate oligomer, an acrylate monomer and a photoinitiator. The composition has a viscosity of between 300 to 600 mPa·s so that it can be applied, for example, by gravure offset printing. Upon curing, the composition provides a polymer material which is slightly self-adhering but does not exhibit any pressure-sensitive adhesive properties.
U.S. Pat. No. 4,174,307 and U.S. Pat. No. 4,305,854 disclose a UV radiation room temperature curable monomeric substantially water-insoluble composition comprising polyurethane compounds having two or more olefinically unsaturated groups, and a photoinitiator. The compositions can be printed and used, for example, as vehicles for printing inks. The shear values of the cured polymers do not meet all practical requirements, however,
It was therefore an object of the present invention to provide a UV-curable solvent-free precursor of a pressure-sensitive adhesive which can be applied by printing techniques such as, for example, screen-printing or flexo-printing and which does not exhibit the shortcomings of the compositions of the prior art or exhibits them to a lesser degree only. It is another object of the present invention to provide such a precursor which can be cured in short curing times of preferably less than 5 minutes and especially preferably of less than 1 minute. It is another object of the present invention to provide such a precursor which upon curing provides a pressure-sensitive adhesive having high values of static shear from various substrates including ABS (acrylonitrile-butadiene-styrene) and/or high shear adhesion failure temperatures (SAFT) from various substrates including stainless steel. Other objects of the present invention can be taken from the following detailed specification of the invention.